NavList:

I think this is still a little off. I could be wrong, of course, but I think you have to move the "subtract from 180" step to a different spot. The sea horizon has "dip" in both directions.

Let's imagine a star with a true (corrected, geocentric, unrefracted, relative to the true horizon) altitude of 72°30.0'. To allow that altitude to remain nearly unchanged for some minutes, we can imagine that the star is crossing the local meridian at this altitude, and to make it specific we can suppose that it's crossing the meridian south of us. Let's suppose we measure the altitude of this star with a marine sextant that has an index error of 1.5' and therefore an index correction of -1.5'. And also suppose a height of eye of 106 feet which implies a standard dip of 10.0'.

First we'll measure the altitude of the star facing south. This is a standard direct altitude, and the result will be just about 72°41.8'. Then we immediately turn around, and less than a minute later we measure the altitude facing north. This will be a "backsight", and the result will be nearly 107°41.2'. Both of these are raw sextant altitudes, straight off the instrument, before any corrections. Notice that the sum of these altitudes is 180°23'. Three minutes of that can be ignored immediately since that's just the index correction on each sight. When cleared (corrected), both altitdues should yield the original true altitude, 72°30.0'. So what are the rules then?

Frank Reed